The switching of electric power has long been a requirement for the operation and control of various systems. These systems include everything from the simple flipping of a light switch to turn on a light or the resetting of a circuit breaker switch which has automatically tripped due to a circuit overload, to the very complex and sophisticated computer controlled switching and load shedding of electric power on the space shuttle. While manually operated electrical switches are adequate for many of these applications, increasingly electronic control is being utilized to effectuate the switching of electric power. Even modern room lighting systems utilize electronic motion sensors to control electrically actuated switches to turn on and off lights within a room.
While small control electronics are well suited for processing the required inputs and performing the required logic to control the switching of the electric power, many of these electronic components operate on digital voltages and currents and are not suitable for the switching of the greater amounts of electric power needed to operate most electrical equipment. While there have been many advances in the development and manufacture of high power switching electronic circuitry, the cost and cooling requirements of these devices, such as IGBTs, MCTs, and MOSFETs, preclude their application in many electric power switching applications. In many of these applications, ranging everywhere from consumer appliances, to electronic wall-mounted hand dryers, to large computer controlled factory equipment, the use of the electronically controlled electromechanical relay provides the required function at a cost and with a reliability which is acceptable.
A typical electromechanical relay, such as that illustrated in FIG. 5, typically comprises at least one, and possibly two drive coils 10. In the case of a single coil relay, the coil 10 is energized to create a magnetic field which pulls a moveable contact electrode 12 into physical contact with a stationary contact electrode 14 to complete the electrical circuit between the two power terminals 16, 18 for a normally open relay. If the relay is of the normally closed type, the energization of the drive coil 10 will create a magnetic field which separates the physical contact of the two contact electrodes 12, 22 thereby breaking the electrical circuit between the two power terminals 18, 20. These single coil relays also typically include a bias spring (not shown) to hold the moveable contact electrode into its quiescent state, i.e. away from the stationary contact electrode 14 for a normally open relay, and in contact with the stationary electrode 22 in the normally closed type relay. Various other designs are available for relays depending upon the particular application requirements. More sophisticated electromechanical relay designs include both a drive open and a drive close coil, requiring the application of an electrical drive signal to both open and close the relay. Other designs include latching type relays which allow the coil current to be switched off once the relay has transitioned, as well as coil cutthroat mechanisms which ensure that both the open and close drive coils are not energized at the same time. Other relay designs provide both normally opened and normally closed contacts, and many provide auxiliary contacts for relay position sensing for feedback control.
Regardless of the particular construction of the actual relay switching element, its reliability will be determined by the number of cycles it will withstand in its lifetime. As one skilled in the art will recognize, the mechanical simplicity and robustness of a typical relay design does not provide the limiting factor which determines the relays life. Instead, the typical limiting factor in a relay's life is a purely electrical phenomenon occurring in most relays upon the opening and closing of the contact electrodes. Specifically, the opening and closing of the contact electrodes results in an electrical arc forming across the contacts for a small period of time. The period of time during which an arc flows is determined by many factors including the mechanical bounce of the contacts upon closure, the distance between the contact electrodes, the magnitude of current flowing, as well as the level of ionization of the air in the gap between the contact electrodes. This electrical arc will also be extinguished, in the case where an AC current is being switched, when the voltage between the contacts traverses through zero and the cycle changes from positive to negative or negative to positive.
The electrical arc between the contact electrodes of an electromechanical relay limit the life of the relay in essentially two ways. First, the electrical arcing leaves carbon deposits on each of the contact electrodes which, over time, build up to form a high resistance contact between the contact electrodes. This high contact resistance results in increased heat dissipation within the electromechanical relay, as well as reduced voltage available at the relay output. Eventually, the material build up on the contact electrode surfaces will result in intermittent contact of the contact electrodes. This intermittent contact results in the electrical circuit not being completed when the relay is energized due to the insulating properties of the build up material which prevent a physical contact of the conductive material of the contact electrodes.
A second way in which the life of an electromechanical relay is shortened by the electrical arc formed between the contacts during opening and closure thereof is a result of the extreme heat of an electrical arc. Specifically, as an electrical arc is drawn between the two contact electrodes, a small portion of the contact electrode material will be melted or vaporized off of the surface. The amount of material burned away during each cycle during which an arc is formed is a function of the voltage and current which the relay is attempting to switch. The higher the current flow between the electrical contact electrodes, the hotter the electrical arc, and thus the more contact material that is burned away. A second factor is the amount of contact material on the surface of the contact electrode. While gold provides a very high fidelity electrical contact, its expense requires that it be plated onto the surface of the contact electrode in relatively thin layers. These gold plated contacts are particularly susceptible to failure from electrical arcs drawn during the switching operation due in part to the small amount of gold which is present and in part because of the softness of gold itself.
An alternate failure mode of electromechanical relays due to the arc generated, primarily during closure of the contacts, is the welding together of the contact electrodes. Specifically, as the contact electrodes come into contact, the force with which they are brought together typically results in a slight mechanical bounce of the two contact electrodes, resulting in multiple contact and separation events in a very short period of time. Each of these bounce events results in the generation of an electrical arc which tends to greatly increase the temperature of the contact electrode surfaces. This particular failure mode is generated when the surface material on the contact electrodes is heated to a sufficient degree to liquify, to some degree, the surface material. If both electrical contact surface materials are liquefied and the contact electrodes are brought into physical contact, these two electrodes will be welded together. Such an event is a latent failure, the existence of which is not known until it is desired to break the electrical contact to de-energize the load to which the relay is connected. At that point it is realized that the relay has been welded in a closed configuration, and the circuit is no longer able to be broken, resulting in the continued energization of the electrical load.
As this problem has existed since the invention of the very first switch, many attempts have been made to overcome this problem. A family of solutions exist whereby an electronic controller attempts to control the physical opening and closing of the electromechanical relay at a point of minimum voltage difference between the electrodes. Specifically, it is desirable to open or close the contact electrodes when the voltage existing on the relay is zero volts (at the zero crossing of AC waveform). However, since actuation of an electromechanical relay requires the physical movement of the contact electrodes, there will be some delay from the initial close command issued by the electronic controller until the magnetic field has built to a sufficient level to begin movement of the contact electrodes by overcoming the spring force. Additionally, there will also be a delay due to the amount of time it takes for the contacts to transition from their fully open to fully closed position.
Prior attempts to measure the contact closure and opening timing have involved the measurement of the voltage across the contacts or the load. However, this method has certain problems including that resulting from the contact bounce on closure. It is a phenomenon of electromechanical relays that as the relay contacts become aged, they tend to have more electrical bounce. This bounce in turn provides false data for contact timing measurement. Other methods to measure contact closure and opening timing include the determination of the nominal contact timing at the time of manufacture of the electromechanical relay, and using this data in the electronic controller as a built-in delay. This method however presents problems as the relay ages and the timing of the opening and closure changes, since there is no means of compensating for the fixed delays stored in the controller. An additional effect on the nominal timing of the opening and closure exists with variations in drive voltage and operating temperature of the environment in which the relay is situated.
Another problem exists with prior controllers in that they do not distinguish between switching during the positive or negative cycle of the AC waveform, nor at the beginning or the end of the AC waveform half cycle. In the first situation, where controllers do not distinguish between switching during the positive or negation half cycle of the AC waveform, plating of metal from one contact electrode to the other may result. While this process may be slowed by the controller which attempts to open and close the contacts near the zero cross point of an AC waveform, the process will still eventually result in failure of the contacts.
The second consideration which prior designs have failed to recognize, that of closing at the beginning or the end of a half cycle of the AC waveform, also reduces the life of the relay over an optimized design. Specifically, since variations in the timing of the relay opening and closing cannot be measured before they occur, the electronic actuation with a built-in delay will likely result in a closure of the contacts (or an opening of the contacts) at a point slightly displaced from the actual zero crossing instant of the AC waveform. If the contact is transitioned such that the opening or closing with bounce occurs at the beginning of a half cycle of the AC waveform to be switched, a small arc may be formed which will increase in intensity as the voltage difference between the electrodes increases at the start of the half cycle, and may last for the entire length of that half cycle (8.333 milliseconds for a 60 hertz AC waveform). On the other hand, if the contacts are transitioned to make or break the physical contact slightly before the zero cross point, the arc which may be generated, in addition to being small to begin with, will be extinguished as the voltage difference between the electrodes continues to fall as the zero cross point is approached.
There therefore exists a need in the art for an electronic controller which overcomes these and other known problems existing in the art which decrease the reliability and lifetime of electromechanical relays.